This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Mammographic breast density is known to contribute to breast cancer risk due to (a) additional hormonal exposer and physiological influence, and (b) obscuring of lesions. Little has been investigated as to how MRI breast density may compare with mammographic breast density. Even less is known about how DOS is influenced by specific MRI signatures in tissues. Using a dedicated DOS instrument on loan to the UCSF Breast imaging center, we have co-registered MRI and DOS measurements. The MRI was used to compute breast density as well as provide a 3D mesh for modeling of light transport in the breast. The goal is to understand both MRI and DOS better by combining techniques to reveal the origins of breast density.